1. Field of the Invention
This invention relates to an electroluminescent device (EL device), and more particularly to an electroluminescent device the light-emitting layer of which has improved voltage resistance and improved current resistance.
2. Description of the Prior Art
An electroluminescent device consists of a basic structure composed of electrodes and a light-emitting layer inserted between the electrodes and formed by adding an active material which constitutes the light-emitting nuclei, such as Mn, Tb and Sm, to a semiconductor, for example, ZnSe and ZnS. In order to generate the light in the EL device, an electric field or an electric current is applied to the light-emitting layer to strike the electrons against the light-emitting nuclei. While a light-emitting layer is formed, defective portions having a low voltage resistance and a low current resistance, such as pinholes and crystal grain boundary occur therein; it is difficult to form a light-emitting layer having no defects with respect to the total surface area thereof. Due to the low voltage resistance and low current resistance which are ascribed to these defects, it is difficult to use the EL device practically with such a basic structure alone. There is an AC EL device adapted to emit light when an electric field is applied thereto, in which layers of an insulating material are formed on both sides of a light-emitting layer to secure a high voltage resistance thereof and thereby solve the above-mentioned problems. However, in a DC EL device adapted to emit light when an electric current is injected thereinto, it is impossible to insert layers of an insulating material between the electrodes. Accordingly, it is difficult in a DC EL device to improve the voltage resistance thereof. Furthermore, a DC EL device has another problem in that it is burnt due to the current concentration caused by the heterogeneity of and defects in the light-emitting layer thereof. Therefore, a DC EL device has not yet been put to practical use.